1. Field of the Invention
The present invention is directed to a method as well as to an apparatus for transmitting data from a rotating part to a stationary part of a system wherein data are generated, and in particular to such a method and apparatus for transmitting data representing radiation attenuation values obtained in a computer tomography (CT) apparatus while rotating a live ring relative to a stationary frame, wherein the data are transmitted from the stationary frame to a computer for generating an image.
2. Description of the Prior Art
Computed tomography (CT) systems are a well-known imaging modality wherein a rotary part, known as a live ring or gantry, carries a measurement system which is rotated around a rotational axis relative to a stationary part of the apparatus. The measurement system typically is formed by an X-ray source and a radiation detector. Power is supplied from the stationary part to the components on the rotary part, typically via slip rings, and the rotary part continuously revolves while X-ray measurement data are being generated and transmitted to the stationary part. The data are typically transmitted from the stationary part to a computer wherein an image of an examination subject, around whom the measurement system rotated, is reconstructed.
A recent development in computed tomography are multi-slice CT systems, which generate larger amounts of data which must be transmitted in a shorter time between the measurement system and the image reconstruction system. For example, a four-slices CT system rotating with a maximum of 120 rot/min requires a data link with a capacity of approximately 200 MBaud. Increasing the number of slices to 16 with the same rotational speed increases the required transmission rate to 800 MBaud. The transmission rate can exceed 1 GBaud if the system rotates even faster. Transferring such data in real time at this rate between the rotary part and the stationary part is difficult to implement with conventional techniques.
Conventional computed tomography systems which do not require such a high transmission rate have employed baseband transmission between a rotating transmitting antenna, formed in strip-line technology, and a stationary receiving antenna mounted relatively close to the transmitting antenna. Such a system is described, for example, in U.S. Pat. No. 5,140,696. In a variation of this conventional arrangement, U.S. Pat. Nos. 4,794,796 and 4,796,183describe systems wherein data are transmitted from a rotary part to a stationary part via a rotating waveguide.
These conventional techniques becomes increasingly difficult to implement when the data transmission rate reaches the GHz range, because the wavelength becomes shorter, and the spacing between the rotary part and the stationary part is then comparable to a quarter of the wavelength. With shorter wavelengths the requirements for mechanical precision and alignment of components become more critical, thereby increasing the overall cost. Moreover, as the data rate increases it becomes more difficult to address the electromatic compatibility (EMC) problems associated both with limiting parasitic radiation and providing immunity to external perturbations.
In recognition of these problems associated with broadband transmission, modulated light has been employed as an alternative transmission medium. Systems employing modulated light for this purpose can be categorized as four basic types, as follows.
In a first approach, a number of rotating light sources are used, which emit overlapping light beams which propagate toward one or more stationary light receivers. For example, U.S. Pat. No. 4,996,435 discloses an optical system for transmitting data between a stationary part and a rotary part having a number of light transmitters arranged on a circle on one of the parts, with a single light receiver disposed on the other part. U.S. Pat. No. 5,229,871 discloses an optical data link for communicating data between a stationary member and rotary member, and an X-ray computed tomography apparatus incorporating such an optical data link, wherein one or more transmitters are used with a single receiver, with elliptical reflectors to maximize the light captured by the receiver. U.S. Pat. No. 5,469,488 discloses an X-ray computed tomography scanner having a number of light-emitting elements and a light-receiving element, with a light collector to converge the light to the receiver.
A typical computed tomography apparatus has a gantry with a relatively large diameter, of approximately 1.5 m. A disadvantage of systems of this first type when employed in such a computed tomography apparatus is that, in the GBaud range of data transmission, the bit duration is less than 1 ns, whereas the signal propagation speed in air is about 3 ns/m. This means that the transmitted signal will be delayed by respectively different amounts from different transmitters on the rotary part relative to the stationary receiver, and this difference is larger than the bit duration and therefore leads to mode divergence and pulse widening. This makes this first approach inappropriate for transmitting data at high rates.
A second known approach is to rotate a modulated light source with the rotary part, and to laterally couple the modulated light into a stationary optical fiber ring. A light receiver is axially coupled to the optical fiber.
This approach is exemplified by German PS 44 21 616, as well as U.S. Pat. No. 6,043,916. In U.S. Pat. No. 6,043,916, a single light emitter is used to transmit the informational signal, and the receiver is a fluorescing fiber optic connection which proceeds within a circular ring configuration, and which has at least one detector mounted at or near at least one of the fibers. The emitter emits the informational signals into the fluorescing fiber optic connection at an angle which is approximately perpendicular to the center axis of the fiber.
A disadvantage of this second known approach is the very limited efficiency of coupling light laterally into the optical fiber. The aforementioned use of fluorescent fiber material improves the coupling, however, the fluorescent effect is relatively slow, and thus limits the light's modulation rate, and thus also limits the maximum data transmission rate. In practice, the rate is limited to approximately 100 MBaud with such a fluorescent optical fiber. It is also known to use plastic (i.e., non-fluorescing) fiber with a special cladding to allow light to be injected into the fiber directly from the exterior into the fiber's core. The coupling efficiency, however, is still very low, and the propagation attenuation for the light within the plastic fiber is very high.
In a third known approach, modulated light from a rotating light source is injected axially into a rotating optical fiber ring. A stationary light receiver detects light that is laterally emitted along the entire fiber length.
An example of this technique is disclosed in U.S. Pat. No. 5,535,033 wherein the optical conductor is formed by a bundle of optical fibers having transparent cladding, so that the optical conductor laterally emits light along its entire length corresponding to the data signals which were coupled into the optical conductor.
Another example of this technique is disclosed in U.S. Pat. No. 4,259,584, wherein a ring of light-conductive material is curved around the center of rotation of a rotary part, and light is emitted onto the surface of this material. The light propagates in the conductive ring substantially along the entire length thereof, and the ring has at least one point at which it is interrupted, at which a light receiver is disposed.
A disadvantage of this third known approach is the high attenuation of the signal along the fiber, both due to core loss and lateral emission. Only plastic fibers with an exposed core can be used, so that the attenuation along the plastic fiber is very high. Further, light attenuation increases exponentially along the plastic fiber because of lateral emission. Because of these disadvantages, this third approach is unsuitable for consideration for use with higher data transmission rates.
A fourth known approach is to form a hollow light channel of one channel half in the rotary part and another channel half in the stationary part with the two channel halves facing each other. Modulated light carrying the data is transmitted from the location of a light emitter to the location of a light detector or receiver with several reflections off of the walls of the light channel occurring therebetween.
A system of this type is described in U.S. Pat. No. 4,555,631, which employs only a single reflecting surface in the form of a hollow cylinder with a mirror interior surface. The light source produces two light beams circulating in opposite directions inside the hollow cylinder, with multiple reflections occurring on the interior cylindrical surface until the light beams eventually exit the cylinder via an escape window, toward the light detector. This system is intended to reduce light attenuation by minimizing the number of reflections between the light source and the light detector, by using the smallest possible grazing angle of the light beam on the interior surface of the cylinder. The use of a small grazing angle, however, means that the launch angle for the light emitted from the light source must be very close to the tangent of the cylindrical surface. This means that the light will be reflected multiple times as it propagates along the circular cross-section of the cylinder until it reaches the detector. Another disadvantage of this known system is that the light detector must be preceded by a relatively complicated and precise optical system, so as to capture the light which emerges almost tangentially, and in both directions, from the cylinder, depending on the relative position between the light source and the escape window. Moreover, the light inside the cylinder follows a polygonal path that rotates together with the light source. Therefore, vertices of the polygonal path are moving on the cylinder surface relative to the capture window, and the light emerging through this window therefore will have a variable intensity and a variable angle of incidence, which complicates the structure and circuitry of the light receiver. In order to catch enough light at the light detector during continuous rotation, moreover, the light beam must be of a relatively large diameter, which increases the mode dispersion and leads to pulse widening.
Another system employing this fourth approach has an arcuate hollow light conductor with reflecting interior walls, with a rectangular cross-section. The light conductor has one-half or part that rotates with the rotary part of the gantry, and another part or half that is fixed to the stator of the gantry. Light beams carrying data are transmitted into this conductor, and are subsequently extracted therefrom after propagating through at least portion of the conductor.
The system disclosed in U.S. Pat. No. 5,134,639 has several disadvantages. There is no discussion therein as to how to address the high dynamic range of the received signal due to losses caused by dispersion and reflections. Additionally, the rectangular-shaped cross-section must be very precisely structured, in order to reduce light scattering. Without such a precise structure, light attenuation is very high because the light may be reflected outside of the hollow conductor through an unwanted gap between the rotating and stationary parts. Additionally, this known system uses a highly divergent light source (LED) that increases the light dispersion, as well as increasing the mode interference, due to the fact that the light rays within the beam bundle follow various and different paths of different lengths, and thus exhibit different propagation times. The data transmission rate of this known system, therefore is limited to approximately 10 MBaud.
One further arrangement is known from German OS 2 113 690, wherein a light conducting channel is formed by two facing half channels, one-half channel being formed in the rotary part and the other half channel being formed in the stationary part. These respective half channels have parabolic cross-sections, and thus each parabolic cross-section has a focal point. A light-emitting diode is disposed at the focal point of one of the parabolic half sections, and a photo detector is disposed at the focal point of the other half section.